Add Itemset to Campaign API

Error Response Code	Error Description
INVALID_FIELD_ITEMSET_TYPE	Supported values for itemsetType are [featured]
INVALID_FIELD_CLASS	Supported values for class are [measurement]
INVALID_INPUT_COMBINATION_ADVERTISER_ID_OR_CAMPAIGN_ID_OR_ITEMSET_ID	Resource not found for (advertiser ID and Campaign ID)/(advertiser ID and itemset ID) combinations